When transmitting data through downlink of an OFDM wireless packet communication system, if each user equipment is informed of how many OFDM symbols are used to transmit a control channel, the user equipment has many advantages in using information of the control channel. Especially, a 3GPP LTE system defines a channel notifying the number of OFDM symbols used for the control channel as a physical control format indicator channel (PCFICH).
More specifically, in the 3GPP LTE system, the PCFICH is expressed as 2 bits indicating three states according to whether the number of OFDM symbols used to transmit the control channel is 1, 2, or 3. The 2 bits are increased to bits through channel coding and then expressed as 16 quadrature phase shift keying (QPSK) symbols through QPSK modulation. The PCFICH is always transmitted only through the first OFDM symbol of a subframe and a mapping method in a frequency region for transmission is as follows.
y(0), . . . , y(3) are mapped to resource elements starting with a position of k=k.sub.0, y(4), . . . , y(7) are mapped to resource elements starting with a position of k=k.sub.0+.left brkt-bot.N.sub.RB.sup.DLN.sub.sc.sup.RB/4.right brkt-bot., y(8), . . . , y(11) are mapped to resource elements starting with a position of k=k.sub.0+.left brkt-bot.2N.sub.RB.sup.DLN.sub.sc.sup.RB/4.right brkt-bot., and y(12), . . . , y(15) are mapped to resource elements starting with a position of k=k.sub.0+.left brkt-bot.3N.sub.RB.sup.DLN.sub.sc.sup.RB/4.right brkt-bot.. Here, a value k.sub.0 for shifting a starting point is k.sub.0=(N.sub.sc.sup.RB/2)(N.sub.ID.sup.cell mod 2N.sub.RB.sup.DL).
The above additions include a modular operation of N.sub.RB.sup.DLN.sub.sc.sup.RB, and N.sub.ID.sup.cell denotes a physical layer cell identification (ID).
In the above mapping rule, y(0), . . . , y(15) denote 16 QPSK symbols, N.sub.RB.sup.DL denotes the number of resource blocks transmitted in downlink, and N.sub.sc.sup.RB denotes the number of resource elements per resource block. k.sup.0 is determined according to the cell ID N.sub.ID.sup.cell which varies with each cell. Starting with k.sup.0, the symbols are dispersed in four frequency regions comprised of 4 adjacent resource elements which are not used for transmission of a reference signal (RS) to obtain a frequency diversity gain over all downlink frequency bands, and then transmitted. The reason why the adjacent resource elements which are not used for transmission of the reference signal are used is that since other control channels are constructed with a resource element group (REG) comprised of 4 adjacent resource elements which are not used for transmission of the REFERENCE SIGNAL, multiplexing of a PCFICH with other control channels can be efficiently performed using the same mapping method.
However, when mapping symbols for the PCFICH according to the above-described mapping method, there may be the following disadvantages.
First, if and N.sub.sc.sup.RB is 12 in a general subframe structure of a 3GPP LTE system, the second and fourth frequency regions among 4 frequency regions are mapped over 2 REGs when N.sub.RB.sup.DL is an odd number.
FIG. 1 illustrates a conventional mapping method in which 4 symbols are mapped to 2 REGs.
As illustrated in FIG. 1, if N.sub.RB.sup.DL is an even number, 4 frequency regions for transmission of a PCFICH are identical to units of an REG for transmission of other control channels. However, if N.sub.RB.sup.DL is an odd number, the second and fourth frequency regions are mapped over 2 REGs. In this case, since an REG is comprised of 4 adjacent resource elements which are not used for transmission of a reference signal, resource elements remaining at both sides of the 4 adjacent resource elements cannot be used for transmission of a control channel, thereby wasting resources.
Second, the above-described mapping rule has a region in which mapping can not be implemented.
A frequency mapping region varies with N.sub.ID.sup.cell ID which is unique for each cell. Hence, cells having adjacent cell IDs may reduce interference caused by transmission of a PCFICH. This N.sub.ID.sup.cell is comprised of 504 values ranging from 0 to 503 according to a current 3GPP LTE standard. When N.sub.RB.sup.DL is an odd number, a region in which frequency mapping for PCFICH transmission can not be performed occurs according to N.sub.ID.sup.cell.
FIG. 2 illustrates the case where a region in which mapping can not be implemented occurs in a frequency region for PCFICH transmission according to N.sub.ID.sup.cell. For example, if N.sub.RB.sup.DL is 25, N.sub.ID.sup.cell is 12, and M.sub.sc.sup.RB is 12, a frequency region allocated to 4 successive resource elements deviates from a frequency region in which a PCFICH should actually be transmitted. In this case, the PCFICH can not be allocated to a corresponding region.